Huseth and Groves* studied the movement of neonicotinoids and found that thiamethoxam is capable of moving through a soil lysimeter (column of soil). In experiments in potato fields, they found increased movement of insecticide after the potato vines were killed. This was true for both foliar treatments and timed release soil treatment. In furrow treatments produced the most leachate prior to vine kill. When the vines are killed, neonicotinoid present in leaves may be released to the soil and the root zone moves much less water and insecticide from the soil making more available for leaching. Their study supports non-point source as a contamination mechanism.
Ideally, pesticide contamination is minimized to low levels. This can sometimes be managed with proper formulation or changes in application. This and other studies raise questions about biological impact of these chemicals in ground water.
*Huseth AS, Groves RL (2014) Environmental Fate of Soil Applied Neonicotinoid Insecticides in an Irrigated Potato Agroecosystem. PLoS ONE 9(5): e97081.